1. Field of the Invention
This invention relates to the fabrication of liquid electrolyte-based cassettes and stacks made via encapsulation of the component parts by a resin or thermoplastic matrix. The present invention provides liquid electrolyte electrochemical cassettes and stacks in which at least one reagent and an electrolyte are circulated within the stack or cassette during operation. Cassettes and stacks of the invention are particularly well suited for use in various electrochemical applications, including fuel cells and flow through batteries as well as ion-exchange applications.
2. Background
Electrochemical cells, and particularly, liquid electrolyte batteries and fuel cells are well known in the art. Liquid electrolyte fuel cells convert chemical energy to electrical power with virtually no environmental emissions and differ from a battery in that energy is not stored, but derived from supplied fuel. Therefore, a fuel cell is not tied to a charge/discharge cycle and can maintain a specific power output as long as fuel is continuously supplied. The large investment into fuel cell research and commercialization indicates the technology has considerable potential in the marketplace. However, the high cost of fuel cells when compared to conventional power generation technology has deterred their potentially widespread use. The cost of fabricating and assembling fuel cells can be significant, due to the materials and labor involved, and as much as 85% of a fuel cell's cost can be attributed to manufacturing. Current applications of liquid electrolyte fuel cells are limited to space and military applications requiring high power output and where cost is of little concern.
A single cell liquid electrolyte fuel cell consists of a gas permeable anode and a gas permeable cathode separated by a ionically conducting liquid layer. Energy conversion begins when the reactants, reductants and oxidants, are supplied to the anode and cathode compartments, respectively, of the liquid electrolyte fuel cell. Oxidants include pure oxygen, oxygen containing gases, such as air, and halogens, such as chlorine. Reductants, also referred to herein as fuel, include hydrogen, natural gas, methane, ethane, propane, butane, formaldehyde, methanol, ethanol, alcohol blends and other hydrogen rich organics. At the anode, the reductant is oxidized to produce charge carriers (e.g. ionic species such as protons or hydroxyl ions which are suitable for use in fuel cell applications), which migrate through the electrolyte to the cathode. At the cathode, the charge carriers react with the oxidant. The overall electrochemical redox (reduction/oxidation) reaction is spontaneous, and energy is released. Throughout this reaction, the liquid electrolyte serves to prevent the reductant and oxidant from mixing and to allow ionic transport to occur. The gas permeable anode and cathode prevent leaching of the liquid electrolyte into either the oxidant or the reductant flow field.
The present invention builds on the fuel cell cassette and related methods of manufacture described in our World Publications WO 02/43173 and WO 03/036747, and International Patent Application PCT US03/12684, all of which applications are incorporated herein by reference.
Briefly, WO 02/43173 details a three-step process for the formation of membrane based fuel cell cassettes which includes the following:
1) Sealing of unused manifold openings/ports on each of the particular flow fields (fuel, oxidant, and coolant). For example, in the case of the oxidant flow field, ports utilized for the distribution of fuel and coolant (on other layers) must be sealed about their perimeter to prevent the mixing of these input streams.
2) Sealing of all the ports within the membrane electrode assemblies (MEA) to prevent the leakage of the reactants within the MEA layers.
3) Layering these components (appropriately sealed as described) within a mold or fixture in a method prescribed by the particular stack design. Once the pieces are assembled within the fixture, a resin is introduced about the periphery. Using resin transfer molding or injection molding techniques, the resin is forced into the edges of the cassette assembly. Once hardened, it provides structural support and edge sealing over the assembly.
The resulting fuel cell cassette is then transformed into a fuel cell stack with the addition of end plates. Such a construction provides appropriate manifolding and a means of compression.
Building on that technology, WO 03/036747 and International Patent Application PCT US03/12684 detail a one step sealing process for the formation of membrane based, fuel cell cassettes which included providing a precut MEA and either bipolar plates or flow fields and separator plates wherein each component of the cassette or stack is precut with manifold openings and channels or openings for introducing a sealant into each component of the stack. The assembled stack is then encapsulated with a resin such that the resin encapsulates a substantial portion of the periphery of the cassette and stack and seals at least a portion of the components adjacent to the sealant channels or openings.
Based on our advances in the fabrication of membrane based electrochemical devices, it would be highly desirable to develop fuel cell stacks and cassettes which operate using a liquid electrolyte instead of an ion conducting membrane, with similar improvements in reliability and further reductions in labor and associated costs.